1. Field of the Invention
The invention concerns solid-state gyrolasers used to measure rotation speeds or angular positions. This type of equipment is used, in particular, in aeronautical applications.
Gyrolasers were developed some thirty years ago and are widely sold and used today. Their operation is based on the Sagnac effect, which induces a difference in frequency Ω between two optical emission modes propagated in opposite directions, called counter-propagating, from a rotating bidirectional ring-shaped laser cavity. Typically, difference in frequency Ω is:Ω=4 A ω/λL where L and A are respectively the cavity length and area; λ is the laser emission average wavelength excluding the Sagnac effect; ω is the gyrolaser angular rotation speed.
The value of Ω obtained through spectral analysis of the beat of the two emitted beams serves to obtain the value of ω very accurately. Electronically counting the beat fringes advancing during a change in angular position gives the relative value of the angular position also extremely accurately.
2. Description of the Prior Art
Manufacturing a gyrolaser requires overcoming a few technical difficulties. The first is linked to the quality of the beat between the two beams, which determines the laser's correct operation. Proper stability and a relative similarity in the intensities emitted in both directions are required for the beat to be correct. But in the case of solid-state lasers, the stability and similarity are not guaranteed due to mode—mode competition, which makes one of the two counter-propagating modes monopolize the available gain, to the detriment of the other mode. The problem of bidirectional emission instability for a solid-state ring-shaped laser may be solved by implementing a counter-reaction loop intended to slave around a set value the difference between the intensities of the two counter-propagating modes. This loop acts on the laser, either by linking its losses to the propagation direction, for instance by means of a reciprocal rotating element, a nonreciprocal rotating element and a polarizing element (patent application No. 03 03645), or by linking its gain to the propagation direction, for instance by means of a reciprocal rotating element, a nonreciprocal rotating element and a polarized emission crystal (patent application No. 03 14598). Once slaved, the laser emits two counter-propagating beams with stable intensities and can be used as a gyrolaser.
A second technical difficulty is linked to low rotation speeds, since gyrolasers only work correctly beyond a given rotation speed. At low rotation speeds, the Sagnac beat signal disappears due to the coupling, also known as locking, of the two counter-propagating modes because of the backscattering of the light of the various optical elements present in the cavity. The rotation speed range in which this phenomenon is observed is usually called blind region and corresponds to a minimum beat frequency of a few tens of kilohertz. This problem is not intrinsic to the solid state: it is also encountered with gas gyrolasers. The most common solution for this second type of gyrolasers is to activate the device mechanically by giving it a forced and known movement that artificially places it outside the blind region as often as possible.